1. Field of the Invention
The present invention relates in general to photovoltaic devices and their method of manufacture. More particularly, the present invention relates to amorphous silicon photovoltaic devices constructed to have an improved efficiency.
2. Description of the Related Art
Photovoltaic (PV) devices are used to convert radiation, such as solar, incandescent, or fluorescent radiation, into electrical energy. This conversion is achieved as a result of what is known as the photovoltaic effect. When radiation strikes a PV device and is absorbed by an active region of the device, pairs of electrons and holes are generated. The electrons and holes are separated by an electric field built into the device.
In accordance with a known construction of solar cells using amorphous silicon, the built-in electric field is generated in a structure consisting of p-type , intrinsic (i-type) and n-type layers (p-i-n) of hydrogenated amorphous silicon (a-Si:H). In PV cells having this construction, the electron-hole pairs are produced in the i-type layer of the cell when radiation of the appropriate wavelength is absorbed. The separation of the electrons and holes occurs under the influence of the built-in electric field, with the electrons flowing toward the region of n-type conductivity and the holes flowing toward the region of p-type conductivity. This flow of electrons and holes creates the photovoltage and photocurrent of the PV cell.
It is important that the semiconductor material used for making a PV device is capable of absorbing and converting to useful electrical energy as much of the incident radiation as possible in order to generate a high yield of electrons and holes. In this regard, amorphous silicon is a desirable material for use in PV devices since it is capable of absorbing a high percentage of the incident radiation relative to other materials used in the construction of PV cells, such as polycrystalline silicon. In fact, amorphous silicon is capable of absorbing about 40% more of incident radiation than polycrystalline silicon.
The mobility (.mu.) x lifetime (.tau.) products of the photo-generated electrons and holes are also important characteristics for the semiconductor material used in a PV cell because small .mu..tau. products result in a greater number of electrons and holes that will recombine before being collected.
The structure of a p-i-n photovoltaic device 100 constructed in accordance with the prior art is shown in FIG. 1. PV device 100 includes a transparent substrate 110; a transparent conductive layer 120; a PV cell 125 having a p-type layer 130, an i-type layer 140, and an n-type layer 150; and a reflective conductive layer 160.
With respect to the above noted p-i-n type amorphous silicon PV cell 100 as presently known in the art, the undoped (intrinsic or i-type) layer 140 between p-type layer 130 and n-type layer 150 is proportionally much thicker than the p-type and n-type layers. The I-type layer 140 serves to prevent the radiation-generated electrons and holes from recombining before they can be separated by the built-in electric field. This structure is generally referred to as "p-i-n" if the radiation is incident on p-type layer 130, and "n-i-p" if the radiation is incident on n-type layer 150. However, the use of amorphous silicon as a photovoltaic material is still hampered by the relatively short lifetime of the photo-generated carriers in the i-type layer 140.
The performance of a PV device can, in part, be characterized by a quantum efficiency (QE), which is defined as the ratio of the number of electron-hole pairs actually collected, i.e., contributing to the generated photocurrent, to the number of incident photons.
The QE decreases as an external forward bias voltage is applied to the device causing a reduction in the built-in electric field. Thus, the ratio of the QE measured at a given forward bias to that at a short circuit condition (zero bias) indicates the effectiveness of the device for collecting photocarriers under a reduced built-in electric field. Furthermore, the QE varies as a function of the wavelength and can be measured as a function of the wavelength of the incident radiation which determines the average location where the electron-hole pairs are generated inside the PV device.
For example, the shorter wavelengths of the incident radiation, such as corresponding to blue light, are typically absorbed in i-type layer 140 near the outermost interface, i.e., the interface first encountered by the incident radiation (the p-i) interface in the p-i-n PV device, and the n-i interface in the n-i-p PV device). The longer wavelengths, such as corresponding to red light, are absorbed uniformly in i-type layer 140 and thus cause a large fraction of photocarriers to be distributed between the midpoint thereof and the innermost interface of the PV device (the i-n interface in the p-i-n PV device, and the i-p interface in the n-i-p PV device). Thus, the longer the wavelength of the incident radiation, the greater the depth within i-type layer 140 at which the radiation is absorbed.
An amorphous silicon p-i-n type PV device having an i-type layer 140 that is not doped typically suffers from poor response to red light because the transport of holes under the weak electric field in the middle of i-type layer 140 is much poorer than that of electrons. Therefore, prior to the present invention, there have been efforts to modify the distribution of the built-in electric field across i-type layer 140 in order to improve the efficiency of the PV device, and in particular, the response to red light.
One known technique modifying for the distribution of the electric field across i-type layer 140 is to uniformly trace dope i-type layer 140 with boron. This technique is disclosed by T. D. Moustakas, H. P. Maruska, R. Friedman, and M. Hicks in their article "Effect of Boron Compensation on the Photovoltaic Properties of Amorphous Silicon Solar Cells" published in Applied Physics Letters, Vol. 43, No. 4, pp. 368-370, on Aug. 15, 1983. Because an undoped i-type layer 140 is normally slightly n-type, the electric field across such an undoped i-type layer is strongest near the p-i junction. By providing a level of uniform boron trace doping, i-type layer 140 becomes slightly p-type, thus moving the strongest region of the electric field nearer to the n-i junction. By adjusting the level of trace boron doping in this manner, the spectral response of the collection efficiency can be altered. However, because uniform boron trace doping results in regions in i-type layer 140 having strong electric fields, other regions of i-type layer 140 have relatively low electric fields.
In accordance with another known technique, low electric field regions in i-type layer 140 are minimized by providing a graded boron doping profile. Such a graded boron doping profile of i-type layer 140 is typically linear and has a decreasing concentration of boron from the p-i interface to the n-i interface. This technique is disclosed by P. Sichanugrist, M. Konagai, and Kiyoshi Takahashi in their article "Modeling and Experimental Performance of Amorphous Silicon Solar Cells with Graded Boron-Doped Active Layers" published in Solar Energy Materials, Vol. 11, pp. 35-44, in 1984. In accordance with this practice, i-type layer 140 is doped with boron with a graded profile extending between the p-i and n-i interfaces such that it is slightly p-type at the p-i interface and slightly n-type at the n-i interface. As a result, the electric fields at the interfaces are reduced and the built-in electric field is more uniformly distributed throughout i-type layer 140. By providing a uniform electric field throughout i-type layer 140, the regions having low electric fields are minimized, as is the likelihood that the electrons and holes traveling through such regions will recombine.
The above described techniques have been adopted based on the belief that the electric field distribution in the i-type layer needs to be modified. However, none of the above techniques based on this belief have resulted in an amorphous silicon PV device with a significantly increased overall efficiency.
Further, the present inventor has observed that the above described techniques involve doping i-type layer 140 proximate the p-i interface which, disadvantageously, results in a decrease in the electric field at the p-i interface. A decrease in the electric field at the p-i interface in a p-i-n type PV device results in a decrease in its response to blue light. This is because blue light is absorbed in the region near the p-i interface and electrons generated by the blue light may not be subjected to a sufficiently strong electric field in that region to cause them to travel the entire width of i-type layer 140 to be collected at n-type layer 150. Thus, although the above described techniques may provide an improved response to red light as compared to an amorphous silicon PV device having an undoped i-type layer 140, the response to blue light and the overall efficiency of the PV device is actually reduced.